1. Field of the Invention
The present invention relates to an optical receiver and an optical transceiver and more particularly, to an optical receiver and an optical transceiver that are useable for transmission of optical signals at a high bit rate over a long distance without chromatic dispersion compensation. The present invention is applicable to optical transceiver modules used for optical transmission systems, network systems, and so on.
2. Description of the Related Art
In recent years, a variety of small-sized optical transmitters has been developed according to the advance of the optical transmission technologies. With these small-sized optical transmitters, there is a major trend to realize higher-speed and longer-distance transmission with technological innovation. At present, a small-sized optical transmitter capable of optical transmission at a bit rate of, for example, 10 Gbps (gigabit per second), over a distance of approximately 80 km using single-mode optical fibers has been developed. However, according to the increase of the bit rate, optical transmission using the ordinary NRZ (Non Return to Zero) modulation method has already reached the transmission limit caused by chromatic dispersion.
Accordingly, a variety of new modulation methods has been developed to overcome such the transmission limit as above so far; however, they have disadvantages. For example, with the Duo Binary modulation method, there is a disadvantage about the size, power consumption, and cost of the optical transmitter. On the other hand, with the Chirp Managed Laser (CML) method, the disadvantage about the size, power consumption, and cost of the optical transmitter has been improved to a certain extent; however, the size and cost of the transmitter need to be lowered furthermore.
The conventional optical transceivers have the following two problems.
The first problem is that the long distance transmission capability (concretely speaking, chromatic dispersion tolerance) is insufficient and therefore, it is difficult to transmit signals at a high bit rate of approximately 10 Gbps over a distance of 100 km or longer without chromatic dispersion compensation. This problem is caused by the following.
Specifically, it is general that the bandwidth of the modulation spectrum of light spreads according to the increase of the bit rate and therefore, the light propagating in the optical fiber transmission lines is more likely to be affected by chromatic dispersion. As a result, the waveform degradation of the light will be prominent, which means that the long distance transmission is limited.
The second problem is that if we seek to solve the above-described first problem about the long distance transmission capability, the size of the optical transceiver becomes large and the power consumption thereof is increased. The cause of this problem is as follows:
If we seek to carry out one of the above-described newly-developed modulation methods in order to reduce the bandwidth of the modulation spectrum of light, the circuit structure of the transmission section is complicated. For this reason, the size of the optical transceiver is enlarged and as a result, large modulation power is required.
Accordingly, to solve the above-described two problems simultaneously, there is the need for an optical transceiver that is capable of signal transmission at a high bit rate of approximately 10 Gbps over a distance of 100 km or longer without chromatic dispersion compensation and that is small in size and low in power consumption.
Next, other related art of the present invention will be explained below.
The Patent Document 1 (the Japanese Non-Examined Patent Publication No. 2004-343360) published in 2002 discloses an optical communication device and an optical communication system for wavelength multiplexing optical communication systems, which enhance the utilization efficiency of the frequency of light easily.
With the optical communication device disclosed by the Patent Document 1, a pair of differential Non return to Zero Inverted (NRZI) signals, which have been cut-off in a wide range, is respectively applied to two branch paths of a push-pull type optical intensity modulator. The amplitude levels of the pair of differential NRZI signals and the DC bias level of the optical intensity modulator are set appropriately. In this way, an inverted Return to Zero (RZ) signal of light whose phases of respective adjoining marks are reversed is generated. Moreover, the bandwidth of the inverted RZ signal of light thus generated is limited by an optical filter to thereby suppress the spectrum width of the said RZ signal. (See Abstract, FIGS. 1 to 4, and paragraphs 0015 to 0031 of the Patent Document 1.)
The Patent Document 2 (the Japanese Non-Examined Patent Publication No. 3-9624) published in 1991 discloses a modulating/demodulating method for an optical communication system, where an optical signal is frequency-modulated at a light source and transmitted on the transmission side, and the said optical signal is subjected to a frequency-amplitude conversion and thereafter, detected on the reception side.
With the modulating/demodulating method for an optical communication system disclosed by the Patent Document 2, the signal light is frequency-modulated by the semiconductor laser of the transmitter, and is transmitted toward the reception side by way of optical fibers and optical repeaters, where the signal light is amplified by optical amplifiers during transmission. On the reception side, for example, the signal light is divided into two parts and then, a phase difference of π is applied to these two parts and combined again, resulting in an amplitude-modulated signal light. Alternately, at the end of the reception side, the frequency-modulated signal light is converted to an amplitude-modulated signal light by appropriate frequency-amplitude converting means (e.g., frequency-amplitude conversion using a Fabry-Perot interferometer) in advance. Subsequently, the amplitude-modulated signal light is directly detected by an optical receiver (e.g., photodiodes). In this way, the amplitude of the signal light received is kept constant. As a result, the waveform distortion of the signal light in the saturation region can be avoided.
In addition, since the spectrum width of the signal light is restricted, the effect of chromatic dispersion of the optical fibers can be decreased. (See claims, FIGS. 1 and 2, “action” section in lower right column of page 2, “advantage” section in lower left column of page 3 of the Patent Document 2.)
The Patent Document 3 (the Japanese Non-Examined Patent Publication No. 6-265832) published in 1994 discloses a tunable optical filter device that makes it possible to convert a frequency-modulated signal light to an intensity-modulated one stably at high efficiency.
With the tunable optical filter device disclosed by the Patent Document 3, an inputted frequency-modulated signal light is converted to an intensity-modulated signal light by an optical filter element having a wavelength transmission characteristic where the central transmission wavelength is variable. The intensity-modulated signal light thus obtained is converted to an electric signal by way of an optical detector and an amplifier. From the output of the amplifier (i.e., the electric signal), the intensity amplitude of the intensity-modulated signal light is detected using a detector. Thereafter, the intensity amplitude signal obtained from the detector is inputted into a CPU (Central Processing Unit), thereby controlling the wavelength transmission characteristic of the above-described optical filter element by a rotation-angle detecting mechanism and a temperature adjuster in such a way that the conversion efficiency from the frequency-modulated signal to the intensity-modulated one is maximized. The above-described optical filter element is a narrow-band tunable filter having a single peak, which may be formed by a Fabry-Perot resonator. (See Abstract, FIGS. 1 and 2, and paragraphs 0016 to 0019 of the Patent Document 3.)
The Patent Document 4 (the Japanese Non-Examined Patent Publication No. 7-38503) published in 1995 discloses a frequency-modulated signal transmission device (FM transmission device), where the demodulation efficiency deterioration is prevented even if the central frequency of a frequency-modulated signal is fluctuated during the demodulation process thereof.
With the FM transmission device disclosed by the Patent Document 4, the subcarrier is modulated by a data signal according to the amplitude shift keying. Moreover, using the said modulated signal according to the amplitude shift keying, the carrier for an optical or electrical signal is frequency-modulated and transmitted. On the reception side, after the reception, the frequency-modulated signal is converted to an amplitude-modulated signal by a self-delaying interferometer and thereafter, the envelope component of the said amplitude-modulated signal is extracted by envelope detecting means. Moreover, the fundamental of a predetermined angular frequency and its secondary harmonics are extracted using a band-pass filter and then, they are envelope-detected. Finally, these signals obtained by the envelope detection are respectively squared by squaring circuits and added to each other. In this way, the data signal is demodulated. (See FIGS. 1 to 4, and paragraphs 0015, 0019 to 0029 of the Patent Document 4.)